1. Field of the Invention
The present invention relates to a projection display apparatus for magnifying and projecting an image formed by an image forming device onto a projection surface such as a screen or the like.
2. Description of the Related Art
FIG. 1 of the accompanying drawings is a schematic side elevational view of an imaging optical system disclosed in JP-A No. 10-111458. As shown in FIG. 1, the disclosed imaging optical system is a reflective imaging optical system comprising three reflecting mirrors, i.e., first, second, and third reflecting mirrors, 101, 102, 103. The image forming device 100 shown in FIG. 1 is disposed on an imaging surface. Image forming device 100 emits light beams representing an image, which are successively reflected by first reflecting mirror 101, second reflecting mirror 102, and third reflecting mirror 103. The reflected light beams are then projected obliquely upwardly onto the reverse surface of a screen (not shown). The reflective imaging optical system disclosed in JP-A No. 10-111458 is will be reduced in size and, in particular, have a profile with reduced thickness, because the paths of the light beams emitted from image forming device 100 are folded back on themselves a plurality of times by the plurality reflecting mirrors.
The light beams (image) emitted from image forming device 100 are progressively magnified as they are reflected successively by first reflecting mirror 101, second reflecting mirror 102, and third reflecting mirror 103. If first reflecting mirror 101 to which the light beams emitted from image forming device 100 are first applied has a distortion or a deformation, the effects of distortion or deformation are also progressively magnified. Therefore, the reflecting surface of first reflecting mirror 101 is required to have a higher level of surface shape accuracy than the reflecting surfaces of second reflecting mirror 102 and third reflecting mirror 103. In addition, first reflecting mirror 101 is highly affected by heat from image forming device 100 because first reflecting mirror 101 is closest to image forming device 100. It is desirable that first reflecting mirror 101 be made of a material having a low linear coefficient of expansion.
For the above reasons, first reflecting mirror 101 of the reflective imaging optical system disclosed in JP-A No. 10-111458 has a spherical reflecting surface. The spherical reflecting surface can be finished to high accuracy by a less costly polishing process even if first reflecting mirror 101 is made of a glass material having a low linear coefficient of expansion.
The paths of the light beams extending from image forming device 100 to the screen have different lengths, respectively, which are responsible for an aberration (trapezoidal distortion) on the screen. In the reflective imaging optical system disclosed in JP-A No. 10-111458, second reflecting mirror 102 and third reflecting mirror 103 have an aberration correcting function. Specifically, each of second reflecting mirror 102 and third reflecting mirror 103 has an aspherical reflecting surface which satisfies given conditions.
The pamphlet of International Publication WO01/011425 discloses a reflective imaging optical system with an enhanced aberration correcting function. Structural details of the reflective imaging optical system disclosed in the pamphlet are shown in FIG. 2 of the accompanying drawings. As shown in FIG. 2, the reflective imaging optical system has optical engine (corresponding to image forming device 101 shown in FIG. 1) 200, first reflecting mirror (corresponding to first reflecting mirror 101 shown in FIG. 1) 201, and assistive lens 202 for assisting in aberration correction, which is disposed between optical engine 200 and first reflecting mirror 201. The reflective imaging optical system also has curved mirror 203 disposed closely above optical engine 200, and two other reflecting mirrors. Therefore, the reflective imaging optical system comprises four reflecting mirrors and a single lens. Curved mirror 203 serves as a mirror for avoiding interference between light beams reflected by first reflecting mirror 201 and optical engine 200.
As described above, in the field of reflective optical systems (or reflective imaging optical systems) for magnifying and projecting an image formed by an image forming device, aberrations caused by different optical path lengths are mainly corrected by the surface shapes of reflecting mirrors of the optical systems. Specifically, the shape of a reflecting surface is optimized to enhance the aberration correcting function. However, although the reflecting mirror to which the light beams emitted from the image forming device are first applied should preferably have a spherical reflecting surface, the aberration correcting function of the spherical reflecting surface is low. As a consequence, the reflecting mirrors at subsequent stages undergo an increased corrective capability burden. To solve for such an increased corrective capability burden, it is necessary to increase the number of reflecting mirrors used or to increase the latitude as to the corrective capability of each of the reflecting mirrors.
Increasing the number of reflecting mirrors, however, invites an increase in the cost and size of the reflective imaging optical system. The reflective imaging optical system is highly advantageous in that it is small in size because the paths of light beams are folded back on themselves a plurality of times by a plurality of reflecting mirrors. If the number of reflecting mirrors is increased, then the reflective imaging optical system necessarily becomes large in size, thus canceling the above advantage. The assistive lens that is inserted in the optical path as disclosed in the pamphlet referred to above also increases the number of optical elements, making the reflective imaging optical system large in size.
For increasing the latitude of the corrective capability of each reflecting mirror, the reflecting mirror needs to have an aspherical reflecting surface or a free-form reflecting surface. If, however, the reflecting surface of each reflecting mirror is complex in shape, then it becomes more difficult and costly to design and to fabricate the reflecting mirror. Furthermore, not only the accuracy of a reflecting surface shape, but also the accuracy of a three-dimensional positional relationship of the reflecting mirrors, i.e., the layout accuracy, affects the aberrational correction. Stated otherwise, no aberration can be corrected if the layout accuracy of each reflecting mirror is low even though each reflecting mirror has an ideal reflecting surface shape. For increasing the layout accuracy of each reflecting mirror, the reflective imaging optical system must have reference points or reference surfaces therein. However, if all the reflecting mirrors of the reflective imaging optical system have an aspherical reflecting surface or a free-form reflecting surface, then it is difficult to provide highly accurate reference points or reference surfaces in the reflective imaging optical system.